1. Field of the Invention
The present invention relates to a magnetoresistance effect element, and particularly relates to an element structure of a magnetoresistance effect element having three magnetic layers.
2. Description of the Related Art
As a reproducing element of a thin film magnetic head, GMR (Giant Magneto-Resistance) elements are widely known, and recently, in order to cope with higher recording density, CPP (Current Perpendicular to the Plane)-type GMR elements, in which sense current flows in a direction that is perpendicular to the film plane of the element, have been used. An element that is a type of CPP-type element and that has attracted attention in recent years as a magnetoresistance effect element with high sensitivity is a TMR (Tunnel Magneto-Resistance) element utilizing the TMR effect.
The TMR element includes a stack forming a magnetoresistance effect film (a MR stack) having a magnetic layer (free layer) whose magnetization direction changes in accordance with an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is fixed with respect to an external magnetic field, and a non-magnetic and non-conductive tunnel barrier layer sandwiched between the pinned layer and the free layer. Further, the MR stack is magnetically shielded by shield layers, which also work as electrodes for supplying a sense current to the MR stack, on both sides thereof with regard to the direction of stacking. The free layer is magnetized into a single domain state by a bias magnetic field from bias magnetic field layers provided for example on both sides of the MR stack with regard to the track width direction. When a sense current is applied to the element in the direction of stacking, electrons pass through the energy barrier of the non-magnetic and non-conductive tunnel barrier layer due to the tunnel effect, and flow from the free layer to the pinned layer (or the other way around). It is known that resistance to the sense current changes in accordance with the relative angle between the magnetization directions of the two magnetic layers which sandwich the tunnel barrier layer. By varying of the magnetization direction of the free layer in accordance with the external magnetic field, the above-mentioned relative angle can change, and accordingly a change in resistance to the sense current can be detected.
By making use of such property of the sense current, the TMR element detects the strength of an external magnetic field, and reads the magnetic data of a recording medium. The reproduction output of the magnetoresistance effect element depends on the magnetoresistance ratio. The TMR element has an extremely large magnetoresistance ratio compared to the conventional GMR element, and thus has an advantage of easily realizing a magnetoresistance effect element with high output.
Meanwhile, from the viewpoint of further improvement of the track recording density, a reduction in the spacing between upper and lower shield layers (a gap between shields) is required, and in order to achieve this, a decrease in thickness of the MR stack is required. However, a significant limitation at this time is the presence of the pinned layer. Since the pinned layer requires that the magnetization direction be firmly fixed without being influenced by an external magnetic field, a so-called synthetic pinned layer is usually used. The synthetic pinned layer includes an outer pinned layer, an inner pinned layer, and a non-magnetic intermediate layer which consists of Ru or Rh and which is sandwiched between the outer pinned layer and the inner pinned layer. Moreover, an antiferromagnetic layer is provided in contact with the outer pinned layer in order to fix the magnetization direction of the outer pinned layer. The antiferromagnetic layer typically consists of IrMn. In the synthetic pinned layer, the antiferromagnetic layer is coupled to the outer pinned layer via exchange coupling so that the magnetization direction of the outer pinned layer is fixed. The inner pinned layer is antiferromagnetically coupled to the outer pinned layer via the non-magnetic intermediate layer so that the magnetization direction of the inner pinned layer is fixed. Since the magnetization directions of the inner pinned layer and the outer pinned layer are anti-parallel to each other, magnetization of the pinned layer is limited as a whole. Despite such a merit of the synthetic pinned layer, however, a large number of layers are required to constitute the TMR element that includes the synthetic pinned layer. This imposes limitation on a reduction in the thickness of the MR stack.
Meanwhile, a novel layer configuration that is entirely different from that of the above-mentioned conventional MR stack has been proposed in recent years. In U.S. Pat. No. 7,035,062, a stack used for the CPP-type element, which includes two free layers and a non-magnetic intermediate layer that is sandwiched between the free layers, is disclosed. In this element, two free layers are exchange-coupled via a non-magnetic intermediate layer due to the RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction. A bias magnetic layer is provided on the side of the MR stack that is opposite to the air bearing surface, and a bias magnetic field is applied in a direction perpendicular to the air bearing surface. The magnetization directions of the two free layers adopt a certain relative angle because of the magnetic field applied from the bias magnetic layer. If an external magnetic field is applied from a recording medium in this state, then the magnetization directions of the two free layers are changed. As a result, the relative angle between the magnetization directions of the two free layers is changed, and accordingly, electric resistance of sense current is changed. By making use of such property, it becomes possible to detect an external magnetic field. Such a layer configuration using two free layers has the potential for facilitating a reduction in the gap between the shield layers, because it does not require a conventional synthetic pinned layer and an antiferromagnetic layer and allows a simplified layer configuration.
The layer configuration using two free layers has also a limitation that, in the initial state in which no bias magnetic field is applied, the two free layers must be coupled such that the magnetizations thereof are anti-parallel to each other. However, in the TMR element, since the non-magnetic intermediate layer is a non-conductive tunnel barrier layer, the RKKY interaction leading to antiferromagnetic exchange coupling cannot in principle occur between the two magnetic layers sandwiching the tunnel barrier layer. In addition, the tunnel barrier layer is thin having a thickness of 1 to 1.5 nm. The result is that the two magnetic layers sandwiching the tunnel barrier layer achieve ferromagnetic coupling which allows the magnetizations to be aligned parallel to each other. Therefore, it is extremely difficult to apply the layer configuration using two free layers to an element utilizing the TMR effect which can be expected as a magnetoresistance effect element having high sensitivity.